SPRUI30H November 2015 – May 2024 DRA745 , DRA746 , DRA750 , DRA756
FID determination by VSYNC skew is a method for field ID determination derived from the analog NTSC and PAL interlaced specifications. Under this method, the sending device will not be providing a FID signal. NTSC has 525 total lines split between two fields. PAL has 625 total lines split between two fields. Each of these interlaced standards support an odd number of lines.
Let’s consider just the 525 active line NTSC signal. For the sake of consistency, let’s call Line 1 the first line of the 2-field pair and Line 525 the last line of the 2-field pair.
A waveform is shown in Figure 9-39. VSYNC is defined to go active at the same time as HSYNC for the first line of the first field in a two-field picture pair. For this first field, VSYNC will go inactive after Line 20.
For the second field, VSYNC will go active in the middle of Line 263 to signal that Line 264 is the start of a vertical blanking interval. When HSYNC for Line 264 arrives, coinciding with the vertical blanking interval for the beginning of the second field, VSYNC has already been active for half of Line 263. For the second field, VSYNC will go inactive midway through Line 282 to indicate that Line 283 is active video. When HSYNC for Line 283 appears, VSYNC has already been inactive for half a line.
By seeing whether VSYNC transitions at the beginning of a line or whether it transitions at the midway point of a line, one can determine whether the upcoming group of lines represents the first field or the second field. The derived FID is shown in dashed lines.
The analog NTSC specification defines the field ID changing part way into the vertical blanking. That is, the first few lines of vertical blanking belong to the previous field and the next several lines of vertical blanking belong to the upcoming field. The VIP_PARSER saves one channel of the entire vertical blanking interval between two active video fields into a single buffer. The hardware does not discriminate between whether the vertical blanking lines belong to the bottom of the previous field or those belonging to the start of the next field. This usage model is consistent with vertical ancillary data capture for embedded sync mode of operation.
Obviously, FID Determination by VSYNC skew cannot be used when framing does not use the VSYNC signal but rather relies on the ACTVID signal instead.